The photochemistry of 1,3,3,4- and 1,3,4,4-tetramethylcyclobutene has been investigated in pentane solution with monochromatic far ultraviolet (185, 193, and 214 nm) light sources, as well as in methanol solution with 214-nm excitation. Photolysis of each of the two isomeric cyclobutene derivatives results in competitive electrocyclic ring opening (yielding mixtures of stereoisomeric dienes in each case), fragmentation to yield propyne and methyl-2-butene, and isomerization to the other cyclobutene isomer. Quantum yields for product formation with 185-nm excitation have been measured in each case by cyclooctene actinometry. The occurrence of the interconversion process is offered as evidence for the intermediacy of cyclopropyl carbenes in the photochemistry of simple cyclobutenes in solution. On the basis of the known chemistry of carbenes of this type, their intermediacy in cyclobutene photochemistry may account at least partially for the formation of the formal (2π + 2π) cycloreversion products from photolysis of simple cyclobutene derivatives.